I pretty much liked everything. I could go on and on...
1) For me, Eps1 didn't hit me at first, but later as i watched it i appreciated it even more for what it is. I guess it was just the transition to the new prequel.
2) Every film that Lucas produces just raises the bar. It has been a progressive masterpiece of CGI and sets the Standard.
3) The incredible music as someone said. So memorable and
perfect for each scene.
4) The incredible content of behind the scenes footage.
In Eps1, we see Lucas Day One in writing. 'Long time ago...etc, etc.
Then after draft of screenplay, he partners with artists to design ships, etc for almost 2 years, etc privately. Very interesting.
In Eps2, behind the scenes, can't recall just now but i know it was incredible.

Eps3, behind the scenes of showing the birth of a scene at screenplay (Lava planet fight with Obi-Wan/Anakin) to the very final post-production. You basically see how the entire scene is made. Just multiply that by 400 and you know how Lucas makes a movie.
5) Each actor/actress/creature/costumes/casting was perfect.
6) DEX rocks! (since nobody mentioned him)